fix: make shell use proper msrv

default.nix

@@ -1,38 +1,35 @@
 {
   lib,
   stdenv,
-  rust-bin, # From overlay
+  rust,
   makeRustPlatform,
   packageToml,
-  rust,
   libiconv,
   darwin,
   python3,
-}: let
-  mkPackage = rust:
-    (makeRustPlatform {
-      cargo = rust;
-      rustc = rust;
-    })
-    .buildRustPackage {
-      pname = packageToml.name;
-      inherit (packageToml) version;
+}:
+(makeRustPlatform {
+  cargo = rust;
+  rustc = rust;
+})
+.buildRustPackage {
+  pname = packageToml.name;
+  inherit (packageToml) version;
 
-      src = builtins.path {
-        path = ./.;
-        inherit (packageToml) name;
-      };
+  src = builtins.path {
+    path = ./.;
+    inherit (packageToml) name;
+  };
 
-			dontUseCargoParallelTests = true;
+  dontUseCargoParallelTests = true;
 
-      cargoLock.lockFile = ./Cargo.lock;
+  cargoLock.lockFile = ./Cargo.lock;
 
-			nativeBuildInputs = [ rust ];
-			buildInputs = (lib.optional stdenv.isDarwin [
-						libiconv
-						darwin.apple_sdk.frameworks.Foundation
-				]);
+  nativeBuildInputs = [rust];
+  buildInputs = lib.optional stdenv.isDarwin [
+    libiconv
+    darwin.apple_sdk.frameworks.Foundation
+  ];
 
-			nativeCheckInputs = [python3];
-    };
-in (mkPackage rust-bin.stable.latest.minimal)
+  nativeCheckInputs = [python3];
+}